I joined Baylor in 2008 following thirteen years at UT Austin's Institute for Geophysics and, before that, two years at Utrecht University in The Netherlands. More recently, I spent the 2004-05 academic year at the University of Puerto Rico, Mayagüez and the Puerto Rico Seismic Network.

Research Interests

 Observational seismology: Lateral heterogeneity in the mantle; structure of Earth's internal discontinuities; anisotropy of the lithosphere and deep mantle, structure of the core-mantle transition zone; structure of the lithosphere from waveform modeling, ocean bottom seismology

SIEDCAR intends to learn whether small-scale, "edge driven" convection in the mantle is a significant, "real world" phenomenon and, if so, to explore its effects on surface geology. Specifically, the investigators wish to determine whether a mantle downwelling is occurring along the eastern edge of the Rio Grande Rift and what effect it has on the tectonics of the Southern Rockies.

The evolution of distinct tectonic provinces in southwestern United States since the Cretaceous, including the Great Plains, the Colorado Plateau, and the Rio Grande Rift, has been linked to flat subduction of the Farallon plate beginning about 80 Ma and then its subsequent foundering beginning about 40 Ma. However, there has been a resurgence in tectonic activity (magmatism, extension, and possibly uplift) during the last 10 Ma in the Southern Rockies. The foundering of the Farallon slab occurred about 40 Ma, so there is not a clear connection between that event and present tectonic activity in the region. Small-scale, edge driven convection is a possible explanation for this renewed activity.

Results from a previous study, called "La Ristra", indicate a downwelling under the eastern flank of the rift (Gao, et al., J. Geophys. Res., 109, B03305, doi:10.1029/2003JB002743, 2004 ; Wilson, et al., Nature, 433, doi:10.1038/nature03297, 2005). However, this anomaly is located near the southeastern edge of the La Ristra line, so resolution is less than optimal. Furthermore, edge driven convection, if it is occurring, should extend to the north and south along the margin of the craton, but the one-dimensional La Ristra could not produce a three-dimensional image to delineate the downwelling's boundaries or to quantify its characteristics.

SIEDCAR's investigators are installing a two-dimensional array of 75 broadband seismographs along the western edge of the Great Plains to supplement and increase the density of USArray's Transportable Array stations, which will traverse the region in 2008-2009. The goal of the deployment will be to obtain quantitative estimates of the size, geometry, location, and density contrast of the feature observed by La Ristra and to model its effects on surface deformation.

Understanding passive margins and the continental-oceanic transition has far-reaching economic and societal implications for the following reasons:

 They hide most of the undiscovered hydrocarbon reserves of the USA, and are excellent sites for sequestering carbon scrubbed from the atmosphere. This economic potential invites collaboration between industry and academia.

 Natural hazards of hurricanes, tsunamis, and rapid subsidence also make it imperative to better understand passive-margin evolution.

 Economic and societal concerns provide natural avenues for explaining the importance of this and other "hypothesis-driven" geoscientific research efforts to US taxpayers and political leaders, especially because much of the US population lives on or near our passive margins. Linkages between fundamental geoscientific research and societal issues are relatively visible and easy to explain to residents of the tectonically active western US; comprehensive studies of US passive margins present a similar opportunity to reach and teach residents of the eastern and southern USA.

We have deployed five three-component broadband stations in a portion of the Gulf Coast Plain for which there is already high-quality active-source reflection data, some vintage refraction data, and a published geophysical model to be tested (Mickus, K., Thomas, W., and Keller, G.R., 2007, Variations in Lithospheric structure along the Ouachita belt and their relations to transform offsets, GSA abstracts with Programs, v. 39(6), 616). This deployment will allow us to (1) apply, validate, and fine-tune receiver function techniques for imaging the crust and upper mantle beneath deep sediments and (2) test a geophysical model proposed for this region. The need for such a pilot experiment arises from two sources:

 The importance of studying passive-as well as active-margins in order to understand continental evolution and to motivate students and the public who do not live near active margins and

The results of the pilot study proposed here will be critical to the design of an effective and cost-efficient proposal to learn how continental and oceanic lithosphere merge and, hopefully, to better reconstruct the extensive tectonic history of this passive margin.

Figure 3: Map of current (red) and planned (white) deployment locations for temporary broadband seismographs. The dashed line indicates the location of a model proposed by Mickus et al. (based primarily on gravity data) that we are testing.

Tsunamis have been recorded in the Caribbean Sea since the 16th Century and evidence for significant paleotsunamis is also found in the sediments of the Netherlands Antilles at 400-500 ybp, 1500 ybp and 3500 ybp. There have been 91 reported tsunamis in the Caribbean basin since Europeans moved to the area, of which 27 events are well documented and caused extensive damage and casualties. The most famous of these events include the Venezuela, 1530; U.S. Virgin Islands and northern Lesser Antilles, 1690; Port Royal, Jamaica, 1692; Martinique, 1755; St. Thomas, 1867; Puerto Rico, 1918; and the Dominican Republic, 1946 tsunamis. A tsunami that followed an earthquake in the Anegada Trough created 6-9 meter high waves that entered the harbors of St. Thomas and St. Croix simultaneously. This event lifted the US Navy ship Monongahela onto a pier at Fredriksted, St. Croix. A repeat of this event now, with a 10-fold increase in population density since 1867, and the presence of several cruise ships in the harbors, petroleum carriers, hotels and beach goers, nearby power plants, petrochemical complexes, marinas, condominiums, and schools is estimated to cause as much as $1 billion in direct costs.

In 2005 and 2007 a consortium of institutions, including the U.S. Geological Survey, the University of Puerto Rico, and three Spanish institutions, conducted a geophysical study of the region to the north and east of the Virgin Islands ("Sombrero Seismic Zone"). This study included two six-month deployments of ocean bottom seismographs to record local and regional earthquakes. These data will be integrated with data from the Puerto Rico Seismic Network (whose stations are located exclusively on islands to the south of the Sombrero zone and are therefore biased and incomplete without offshore complements) to identify, locate, and estimate focal mechanisms for small-magnitude seismic events.

Our study will help identify crustal features, including a possible rupture in the subducting North American plate, that could cause tsunamigenic events and will clarify the tectonics that lead to such events. One result of this study will be an estimate of location errors and magnitude bias in routine PRSN event locations, which will improve the reliability of PRSN bulletins in the future.

Figure 1: (from Uri ten Brink) View of the Northeast Caribbean from the east. The Puerto Rico Trench-the deepest part of the Atlantic Ocean and the most extreme negative gravity anomaly on Earth-is located at the center (in purple). Our study area is to the north and east of the Virgin Islands.

Figure 2: The location of the subducting North American tectonic plate inferred by fitting surface to earthquake hypocenters relocated by Engdahl, van der Hilst and Buland (1998). Islands are indicated by red lines, with the Lesser Antilles in the foreground, then the British and U.S. Virgin Islands, Puerto Rico, and Hispaniola. The view is from the southeast.

Green and light blue hypocenters are deemed to be associated with westward and southward-dipping subduction beneath the Caribbean plate. Dark blue hypocenters are believed to be associated with generally northward-dipping subduction of the Caribbean plate beneath Puerto Rico and Hispaniola.

Figure 3: The same surface as in Figure 2 is shown here in copper hues. Red circles are hypocenters relocated with data from a set of nine ocean bottom seismographs in addition to data recorded by the Puerto Rico Seismic Network.